the biggest clearable stair set where i live is 12 but you have to land flatbottom on dirt road. it was a bad idea but a cleared it. now my back wheel has a small hop to it. (i was on my kona which has 6 inches of travel and it was nice and hrd when i did it...still bottomed HARD...won't do it again. still fun though.)
Probably a long 8 was my biggest then I realized how much stair jumping hurt.
As for ones I've witnessed, one was an 11 flat 11 that he landed about mid way down the second 11 set. Worst one by far was (if memory serves correct) a 15 or 20 stair to flat on a first year 250 with a Jr.T. Oh the days when your ability was measured by how high you were willing to drop.